CMOS imagers have been increasingly used as low cost imaging devices. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits is beneficial in many digital imaging applications such as, for example, cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems and star trackers, among many others.
In a conventional CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by a source follower output transistor. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate.
Exemplary CMOS imaging circuits as well as detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,204,524 to Rhodes, U.S. Pat. No. 6,310,366 to Rhodes et al. and U.S. Pat. No. 6,326,652 to Rhodes, the disclosure of which are incorporated by reference herein.
Since prior CMOS imagers suffer from poor signal to noise ratios and poor dynamic range as a result of the inability to fully collect and store the electric charge collected by the photosensitive area, storage capacitors have been proposed for use in connection with the light sensitive node of a CMOS pixel sensor cell to improve collected charge storage. For example, U.S. Pat. No. 6,204,524 to Rhodes describes in detail the formation of planar and trench storage capacitors electrically connected in parallel to the light sensitive node of a CMOS pixel sensor cell and formed partially over the field oxide region and partially over the active pixel region.
Applicants of the present invention have discovered that storage capacitors may also provide useful results when electrically connected to other light sensitive and/or electrical elements of the pixel sensor cell, such as transistor gates or floating diffusion regions, for example, to affect the operation and characteristics of such various light sensitive and/or electrical elements. Capacitors connected to such various light sensitive and/or electrical elements of the pixel sensor cell help amplify the signal of an imager transistor, increase the storage capacitance of a photosite, or provide a low noise decoupling capacitor. Such capacitors may be formed entirely over the active area of the pixel sensor cell, or entirely over the field oxide area, or over both the active area and the field oxide area.